a) m(H₂C₂O₄) = 15.5 g.
n(H₂C₂O₄) = m(H₂C₂O₄) ÷ M(H₂C₂O₄).
n(H₂C₂O₄) = 15.5 g ÷ 90 g/mol.
n(H₂C₂O₄) = 0.172 mol; amount of substance.
N(H₂C₂O₄) = n(H₂C₂O₄) · Na (Avogadro constant).
N(H₂C₂O₄) = 0.172 mol · 6.022·10²³ 1/mol.
N(H₂C₂O₄) = 1.037·10²³; number of molecules.
b) In one molecule of oxalic acid there are two atoms of carbon:
N(C) = 2 · N(H₂C₂O₄).
N(C) = 2 · 1.037·10²³.
N(C) = 2.074·10²³; number of carbon atoms.